The invention relates to a method of controlling a square-wave current light source in a projection system having a color wheel with differently colored segments, which square-wave current is controlled to constantly change the polarity of the square-wave current, to provide a constant power to the light source from polarity change to polarity change, and to provide each time before changing polarity, a current pulse driving the square-wave current to a predetermined current strength which is layer than the current strength in between the current pulses.
The invention also relates to a projection system comprising a square-wave current light source and a color wheel with differently colored segments comprising means for controlling the square-wave current to constantly change the polarity of the square-wave current, to provide a constant power to the light source and to provide each time before changing polarity, a current pulse driving the square-wave current to a predetermined strength which is larger than the current strength in between the current pulses.
Light outputs from such light sources are proportional to the power dissipated by the light source. Therefore, such light sources are power-controlled to control their light output. The power control ensures that the amount of power dissipated by the light source is constant in time. The voltage across the light source is a lamp-dependent constant. This constant, however, increases when the lamp ages.
A known and already resolved drawback of such a light source is that the stability of the arc is not constant with respect to time. Improvement of the arc stability is important to reduce intensity variations of the image and has already been achieved by superimposing a current pulse on the square-wave current for the light source and synchronizing the current pulse with the color wheel. The known superimposed current has a predefined constant value and is set by the lamp supply.
It is also known that light sources for projection systems generally do not have a spectral distribution which accomplishes a white color point perceived as white by a viewer of an image projected by the projection system.
In the prior art, various solutions have been proposed to arrive at an acceptable white color point using square-wave current light sources. Amongst these solutions are those in which the current strength of the current through the light source is increased during the period in which the color of the color wheel is present, which color is less present in the spectrum of the light source.
It has been proposed before to have the current pulse, which is present to improve the arc stability of the light source, coincide with the segment of the color wheel having the color that is less present in the spectrum of the light source. In this way, both the arc stability of the light source is increased and the white color point of the projection system is shifted to a more acceptable value.
It has now been found that, during usage of the light source, a shift of the white color point takes place. The shift is due to the fact that the voltage across the lamp increases with the usage of the lamp. Consequently, when the lamp has aged, a lower current is sufficient to create the same power level as the power level created when the lamp was still new. Another consequence is that the amount of power dissipated during the superimposed current pulse of constant value increases with the usage of the light source, because the lamp voltage increases and the superimposed current pulse remains at the same current level.
Since, however, the total power supplied to the light source from polarity change to polarity change is constant, an increase of the power during the current pulse is compensated by a decrease of the power in between the current pulses. Since, during usage, the voltage across the light source increases, it is not possible to maintain the current strength in between the current pulses at the level used shortly after putting the light source into use. When the voltage increases, there is only one way to decrease the power in between the current pulses and that is to decrease the current strength in between the pulses. Consequently, the intensity of the color during which the current pulse occurs increases, but the intensity of the other two colors decreases slightly. This is visible in the image as a shift of the white color point with the usage of the lamp.
It is an object of the present invention to provide a method of controlling a square-wave current light source in a projection system in which the above-mentioned disadvantage of a shift of the white color point is avoided.
It is also an object of the present invention to provide a projection system with a square-wave current light source which is not subject to the above-mentioned disadvantage of a shift of the white color point with increasing usage of the light source.
To this end, method according to the invention is characterized by controlling the timing of occurrence of the current pulses to constantly coincide in time and, on average, an equal number of times with all colors of the color wheel.
A projection system according to the invention is characterized by control means being adapted to control the timing of occurrence of the current pulses to constantly coincide in time and, on average, an equal number of times with all colors of the color wheel.
It is thereby achieved that the current pulse successively occurs in e.g. a red segment, a green segment and a blue segment. Together, these three pulses lead to white, irrespective of the usage of the light source.
A preferred embodiment of a projection system according to the invention is characterized by a color wheel comprising red, green and blue color segments separate from other red, green and blue segments, respectively, and having widths, which are matched to the durations of the current pulses.
It is thereby achieved that the color wheel comprises a set of segments which is specifically dedicated to the use of the current pulses in a projection system according to the invention.
A further preferred embodiment of a projection system according to the invention is characterized in that the timing control means are adapted to provide equal time periods between successive current pulses, and in that the pulse width-matched color segments are distributed evenly along a circumferential direction of the color wheel.
A still further preferred embodiment of a projection system according to the invention is characterized in that the timing control means are adapted to provide different time periods between successive current pulses, and in that the pulse width-matched color segments are distributed unevenly along a circumferential direction of the color wheel.
It is thereby achieved that, besides the segments whose widths are matched to the durations of the current pulses, the remainder of the color wheel may be divided in a to red, a green and a blue segment, which segments relative to each other do not have the same dimensions in the circumferential direction. As is known from the prior art, a color white point may thereby be achieved that takes an uneven spectral distribution of the light source into consideration.